Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more carriers each defining one or more ranges of frequency spectrum and having a respective downlink channel for carrying communications from the base station to UEs and a respective uplink channel for carrying communications from the UEs to the base station. Such carriers may be frequency division duplex (FDD), in which the downlink and uplink channels are defined as separate respective ranges of frequency, or time division duplex (TDD), in which the downlink and uplink channels are defined on a common range of frequency but are distinguished through time division multiplexing. Further, the downlink channel and uplink channel of each carrier may also be divided into respective sub-channels for carrying particular communications, such as one or more control channels for carrying control signaling and one or more traffic channels for carrying application-layer data and other traffic.
For instance, in a system operating according to an orthogonal frequency division multiple access (OFDMA) protocol, such as the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS) for example, the air interface is divided over time into frames and sub-frames each defining two slots, and the uplink and downlink channels are each divided over their frequency bandwidth into sub-carriers that are grouped within each slot into resource blocks. When a UE is positioned within coverage of a base station in such a system, the UE may register or “attach” with the base station on a particular carrier on which the base station is configured to provide service, and the base station then schedule particular downlink and uplink resource blocks on that carrier to carry data communications to and from the UE. Further, the base station and UE may modulate their air interface data communications at a coding rate selected based on quality of the UE's coverage, such as with higher rate coding rate when the UE is in better coverage of the base station and with a lower coding rate when the UE is in worse coverage of the base station.
In such an LTE system, for instance, when the base station has data to transmit to a UE, the base station may assign certain downlink resource blocks (and thus certain sub-carriers) in a given sub-frame for use to carry the data to the UE at a particular coding rate, and the base station may then (i) transmit to the UE in that sub-frame a downlink control information (DCI) message that specifies the assigned resource blocks and coding rate and (ii) encode and transmit the data to the UE in the assigned resource blocks in that sub-frame. Per the DCI message, the UE would then read the transmitted data from the assigned resource blocks. Likewise, when the UE has data to transmit to the base station and accordingly transmits a scheduling request to the base station, the base station may assign certain uplink resource blocks in a given sub-frame for use to carry the data from the UE at a particular coding rate and may transmit to the UE, in advance of that sub-frame, a DCI message that specifies the assigned resource blocks and coding rate. And the UE may then encode and transmit the data to the base station in the assigned resource blocks in that sub-frame.
A base station may serve a UE on a single carrier, such as the carrier on which the UE attaches with the base station, in which case the base station may allocate resources on that carrier for use to carry communications to and from the UE. Alternatively, the base station may serve the UE on multiple carriers at once, providing “carrier aggregation” service. With carrier aggregation service, the base station serves the UE concurrently on multiple carriers from either contiguous frequency bands or non-contiguous frequency bands, in which case the base station may allocate resources on those multiple carriers at once to carry communications to and from the UE. Further, one of the carriers may be deemed to be a primary carrier or primary cell (PCell) and each other carrier may be deemed to be a secondary carrier or secondary cell (SCell). The PCell and each SCell could then be used to carry communications to and from the UE, and the PCell could also function as a primary transport for certain control signaling between the base station and the UE.